An oscillator is a circuit that generates an oscillating output signal (e.g., that alternately transitions between logic high and logic low states). Oscillating signals are used in a wide variety of applications including, for example, generating clock signals for controlling the timing of digital circuits and generating carrier signals that can be processed to transmit information (e.g., RF signals). A voltage-controlled oscillator (VCO) is an oscillator having an output signal whose oscillation frequency can be controlled by a voltage signal. For example, varying the DC voltage of the control signal can adjust the oscillation frequency of the VCO's output signal, while modulating the control signal can change the frequency modulation (FM) and/or phase modulation (PM) of the VCO's output signal.
Oscillators are typically categorized into two classes depending upon the waveform of their output signals. Harmonic oscillators generate a sinusoidal waveform, and typically include an amplifier and a resonant feedback circuit. Oscillation occurs at the resonant frequency for which a positive gain arises around the loop. Commonly known harmonic oscillators include crystal oscillators and LC-tank oscillators.
Relaxation oscillators generate a sawtooth or triangular waveform, and are typically implemented in monolithic integrated circuits (ICs) because they can provide a wide range of operational frequencies with a minimal number of external components. Commonly known relaxation oscillators include grounded-capacitor VCOs, emitter-coupled VCOs, and delay-based ring VCOs. For the delay-based ring VCOs, which typically include a number of delay stages arranged in a ring, the output frequency is a function of the delay in each of the gain stages. More specifically, ring oscillators typically include an odd number of delay stages connected in a loop such that the output of the last delay stage is coupled to the input of the first delay stage, thereby causing the output of the last delay stage to toggle or oscillate at a frequency proportional to the number of delay stages and/or the delay period associated with each delay stage.
Although conventional relaxation oscillators can provide oscillating signals, it would be desirable to simplify the architecture of the oscillator and/or to reduce the number and complexity of the oscillator's components.